Proponents of materialistic evolution like to talk about how Intelligent Design is not a theory because it is not "falsifiable" (using Anthony Flew's definition, ironically), but evolution is "falsifiable." The idea here is that if there is a path to show something is false (whether it's actually false or not is a different matter), then it can be considered to be a theory.
To illustrate falsifiability, examine the statement, "All car tires are completely black." This statement can be considered a theory because it is falsifiable by counterexample: As soon as someone can show that some car tire is not thoroughly black, you can be certain the statement is false. Though the existence of a pink car tire is a sufficient counterexample, it is not a necessary counterexample. A whitewall tire will do, as will a blackwall with white lettering.
Now, on the subject of origins, the theory, i.e., the thing that can be falsified, is "evolution," and the folks who talk this way are right as far as they go. However, in saying this, they are comparing unlike things: "Intelligent Design" is not a theory, but a philosophical predisposition, while "evolution" is just a family description of a particular set of mechanics, a family of theories. That is, each member of the set "evolution" can be falsified, and so fits Flew's definition of theory.
However, there is something behind "evolution" that cannot be falsified, and is not therefore a theory. For the sake of giving it a name, I'm going to call it "Evolution." This different thing, Evolution, is not a theory, but a predilection to look at any set of facts thrown at it in a particular way. That is, Evolution is a philosophical predisposition. And almost everyone, proponent and detractor alike, confuses the two.